1. Field of the Invention
The present invention relates generally to an image transfer system, and more particularly to a technique for improving the durability of an exposure device of the system for exposing a photosensitive medium to a radiation.
2. Discussion of the Prior Art
There is known an image transfer system using a photosensitive material as a recording medium, on which images are formed with a density that is varied in a certain range according to a variation in an amount of light exposure of the photosensitive material. The lower limit of the range in which the density is variable is determined as a basic minimum amount of light exposure necessary to produce an image visible to the unaided eye. In this type of an image transfer system, images are transfered to the photosensitive medium by exposing the medium to a radiation in response to appropriate image signals. An example of such an image transfer system is schematically illustrated in FIG. 5. In the figure, reference numeral 90 generally indicates a CRT (cathode ray tube) which has a faceplate 92, a fluorescent screen 94 disposed inside the faceplate 92, and an electron gun 96 which produces a beam of electrons 98 toward the fluorescent screen 94. The electron beam 98 emitted by the electron gun 96 is deflected or swept by a deflection coil 100 within a predetermined angular range in a horizontal plane. The intensity of the beam 98 is controlled by a driver circuit (not shown). The fluorescent screen 94 emits a horizontal scanning line of light when struck by a corresponding horizontal line of the electron beam 98 that represents a line of picture elements. Usually, a multiplicity of such horizontal lines correspond to a certain image or a line of images.
The horizontal line of light emitted by the fluorescent screen 94 is converted by an optical filter 102 into a light having a wavelength band to which a photosensitive paper 104 is sensitive. Then, the light is focused on the photosensitive paper 104, by an optical focusing device 106. The photosensitive paper 104 is fed on a support 108 by feed rollers 110, at a speed corresponding to a rate at which successive lines of light are emitted by the fluorescent screen 94. In this manner, latent images are formed on the photosensitive paper 104. If silver chloride is used for a photosensitive material of the photosensitive paper 104, the exposed paper 104 is fed through a developing device 112 for development of the latent images with a developing liquid. If the photosensitive paper 104 is a light- and pressure-sensitive paper which includes microcapsules made of a radiation-curable resin, the image-wise exposed paper 104 is fed through a pressure nip between pressure rollers 114 as a developing device.
In a commonly used photosensitive paper, the density of images formed on the paper is varied according to an amount of exposure to a radiation, only after the paper is exposed to a predetermined basic minimum amount of radiation. For example, in the case of a silver chloride photosensitive paper, the image density is varied within a range B indicated in FIG. 6. In the case of a light- and pressure-sensitive paper using microcapsules made of a radiation-curable resin, the image density is varied within a range B indicated in FIG. 7. In both cases, the image density is changed after the photosensitive medium is exposed to a certain amount of light exposure, i.e., basic minimum amout E1. Accordingly, the formation of latent images on the photosensitive medium requires the fluorescent screen 94 of the CRT 90 to emit a radiation whose intensity must cover the entirety of the density-variable range B, the lower limit E1 of which is considerably high. In other words, the light source for exposing the recording medium, i.e., the CRT 90, must have a minimum luminance capacity equivalent to the basic minimum light exposure E1. Hence, the light source must have a relatively large capacity, and is relatively less durable in operation.
In the case where the above-indicated image transfer system is adapted to form color images, a color balance of the images may be unfavorably changed due to a difference in sensitivity of three types of microcapsules to respective radiation wavelengths corresponding to the three primary colors of light, and/or a fluctuation in the operating characteristics of the corresponding three light sources due to a variation in line voltage and/or ambient temperature. Accordingly, the conventional color image transfer system must be adjusted for consistency of the color balance before the system is shipped. This adjustment takes considerable time and labor, increasing the cost of the system. The color balance adjustment requires control circuits or devices for changing the intensities of the light sources for the three primary colors of light, and/or the relative lengths of times of irradiation of the three different wavelengths. Such control circuits and devices also push up the cost of the system.